Continental extension of northern Gondwana margin in the Eastern Himalaya: Constraints from geochemistry and U--Pb zircon ages of mafic intrusives in the Siang window, Arunachal Himalaya, India.

We report new U--Pb zircon age and whole-rock geochemical data from the Pangin mafic intrusive rocks of the Siang window, easternHimalayas. These mafic rocks are medium to coarse-grained gabbros, consisting mainly of plagioclase and clinopyroxene with accessory phases (hornblende Å Fe--Ti oxides) that retain granular and interlocking texture. Geochemically, they display enriched-mid oceanic ridge basalt (E-MORB) affinity characterized by moderate to slightly fractionated REE patterns marked by (La/Yb)N Æ 2.65¡3.99. Their geochemical characteristics suggest that the parental magmas of these rocks were formed by medium to higher degrees (»12--28%) of partial melting similar to that of the asthenospheric mantle in the garnet-spinel transition zone. Magmatic zircons from two gabbros yield U--Pb ages of 521.50 § 2.53 Ma and 568 § 2 Ma. This new age reveals two pulses of Late Neoproterozoic and Early Cambrian mafic magmatism that are inconsistent with the temporal distribution of Paleozoic magmatism in the Siang window of the Eastern Himalayas. However, based on the results of this study and the correlation of continental extensional mafic magmatism in the Northwest Himalaya, we suggest that investigated mafic intrusive rocks might have been generated in an extensional tectonic environment during the long-lasting Pan-African orogenic cycle of the late Neoproterozoic to early Cambrian which ended with the formation of the Gondwana supercontinent. [ABSTRACT FROM AUTHOR]